1. Field of the Invention
The present invention relates to a material level probe having a pressure seal formed by crimping or swaging the probe with sufficient force to deform the ground, shield, and active elements and insulators such that multiple seals are created in a substantially uniform manner around the circumference of the probe. The probe of the invention is preferably used to measure the level of a material in a storage vessel by mounting the probe on the storage vessel and measuring capacitive and/or admittance differences among the probe elements due to the material in the container.
2. Description of the Prior Art
The present invention addresses the problem of obtaining and maintaining an inexpensive pressure seal on a level measuring probe or sensing element. In the prior art, level measuring probes typically have been sealed against pressure by compressing a bushing into a gland using a compression plug to form a seal against the conductor or conductors that form the probe. A level measuring probe having a seal of this type is shown in FIG. 1.
As illustrated in FIG. 1, conventional level measuring probes comprise a conductive measuring element 10 formed of materials such as carbon steel, stainless steel and the like which is enclosed within an insulator 12 formed of an elastomeric or polymeric insulating material. In the three terminal type probe structure shown in FIG. 1, the measuring element 10 and insulator 12 are telescopically disposed within a guard electrode 14 which is, in turn, surrounded by a second insulator 16 formed similar to insulator 12. As shown, elements 10-16 are preferably telescopically disposed such that measuring element 10 and guard electrode 14 are exposed yet insulated from each other. As shown, guard electrode 14 extends through to the proximal end of the probe where it is connected to a lead 18. A ground to the vessel in which the probe is mounted is provided at ground electrode 20 of a gland 22 as shown.
As known by those skilled in the art, level measuring probes of the type illustrated in FIG. 1 are often mounted in containers under high pressure and are accordingly pressure sealed to prevent leakage of the potentially hazardous substances in the containers. For this purpose, gland 22 is provided for accepting a Teflon bushing 24 which is forced tightly into the gland 22 by a compression plug 26. Elements 22-26 together provide a tight pressure seal against leaks among guard electrode 14, insulation layer 16, and ground electrode 20. A tight pressure seal is similarly formed among measuring element 10, insulator 12, and guard electrode 14 by inserting a screw 28 into the proximal end of measuring element 10, surrounding screw 28 by a bushing 30, and then compressing bushing 30 into the spaces between measuring element 10 and guard electrode 14 using a compression plug 32.
A pressure seal of the type illustrated in FIG. 1 works quite effectively but requires costly machined parts such as gland 22, compression plugs 26 and 32, and screw 28 in order to provide the necessary pressure seal. Also, the compression plugs must be readjusted after the probe has been subjected to extreme temperature variations if a tight seal is to be maintained. It is desired to eliminate the costly machined parts and bushings used in such seals in order to provide a less expensive level measuring probe which need not be periodically readjusted.
One known mechanism for providing a pressure seal between the probe and the guard electrode without using machined glands and plugs is described by Fleckenstein in U.S. Pat. Nos. 4,499,641, 4,549,245 and 4,811,160. Fleckenstein therein describes a molded level measuring probe in which the desired pressure seal is obtained by injection molding the insulation material so that it forms an integral piece which surrounds the rod-shaped measuring element between the measuring element and the guard shield. The effects of shrinkage of the insulation material after the injection molding is minimized by injection molding a lip at a distal end of the probe which radially overlaps the distal end of the guard shield so that the guard shield edge is squeezed during cooling of the insulation material to form a tight, pressure resistance seal between the guard shield and the measuring element. A notch is also formed in the measuring element so that the insulating material filling the notch may form a shoulder which holds the measuring element in its axial position.
However, it has been found that the lip at the distal end of the probe alone does not provide a sufficient seal when the probe is subjected to extreme heating and subsequent cooling. In other words, because the insulation material expands during heating and generally does not return to its exact shape upon cooling, leakage paths are formed between the guard shield and the insulation layer, even at the lip. Nevertheless, the Fleckenstein probe maintains a pressure seal because any material entering the probe through such a leakage path has no path out the proximal end of the probe. In other words, because the guard shield does not extend the length of the probe, there is no leakage path along the guard shield out the proximal end of the probe. Unfortunately, however, since the guard shield does not extend the length of the probe, material which enters the probe or even air gaps formed within the probe as a result of heating and cooling cycles will get between the guard shield and measuring element so as to adversely affect the measured capacitance and admittance between the guard shield and the measuring element.
An improved pressure seal configuration is thus desired which provides an improved pressure seal between the guard electrode and the measuring element and also provides an improved pressure seal between the ground electrode and the guard electrode or, in the case of a two terminal probe, between the ground electrode and the measuring element. A pressure seal is also desired which holds the elements of assembled or molded probes together while sealing all of the potential leakage paths formed by heating and cooling cycles of the probe without affecting the probe's measurements. The present invention has been designed for this purpose.
Fleckenstein observed that an improved seal between the guard electrode and the probe could be formed by crimping or squeezing the tip of the guard shield around the insulation layer between the guard shield and the measuring element so that leakage paths between the guard shield and the measuring element may be substantially closed. However, as also noted by Fleckenstein, crimping of the guard shield tip has heretofore been undesirable since it requires an extra assembly operation during manufacture of the level measuring device and has not heretofore been entirely effective. As a result, the beneficial uses of crimping, swaging, and the like for providing pressure seals in level measuring devices has not been adequately explored. The present invention has been designed to overcome this shortcoming in the prior art and to provide an inexpensive yet effective pressure seal consistent with the above objectives.